This invention relates to an output signal detector of a magnetic memory device capable of gaining a sufficient margin of the logical conversion for one bubble output signal.
In a magnetic bubble memory device, magnetic bubbles are generated in a magnetic bubble memory element made up of a thin magnetic film so as to store information according to the presence or absence of the magnetic bubbles. The detection of the magnetic bubbles, that is, the bubble output is made by applying a rotating magnetic field to the magnetic bubble memory element to move the magnetic bubbles along a propagation pattern, causing the magnetic bubbles to pass by a detector provided at a portion of the magnetic bubble memory element, causing the resistance value of the detector to vary, and detecting such a resistance variation.
FIG. 1 shows one example of a conventional output signal detector of a magnetic bubble memory element 1 comprising a magnetic film. A detector 2a is provided along a propagation path, not shown, of the magnetic memory element and a dummy detector 2b is provided at a portion of the magnetic bubble memory element 1, one end of the respective detectors 2a and 2b being grounded. Constant current sources 3a and 3b are provided for passing constant currents for detection of the resistance variations of both the detectors 2a and 2b. There are also provided a preamplifier 4 for amplifying a variation in the potential across the detectors 2a and 2b corresponding to the variation in the resistance values of the detectors 2a and 2b, a DC regenerating circuit 5 which, in response to an external DC regeneration control signal W.sub.ST, DC regenerates a bubble detection signal variation from an amplified potential variation, and a discriminator 6 which, in response to an external strobe pulse N.sub.ST, converts a DC regenerated signal into a "1" or "0" logical signal.
In the magnetic bubble detector having a construction as described above, the bubble output signal superimposed on a noise waveform N as shown in FIG. 2 is detected and converted into a logical output signal. In this case, one bubble output signal tends to undergo phase shifts due to the variation in the intensity of the rotating magnetic field, a minute change in the magnetic body utilized as the detector, and variation in the manufacturing steps, resulting in a waveform S.sub.1, S.sub.2 or S.sub.3 as shown in FIG. 2. Therefore, the bubble output signal detection must cover any one of the waveforms S.sub.1, S.sub.2 and S.sub.3 in order to prevent failure of detection of one bubble output signal. Thus, a DC regeneration control signal W.sub.ST as shown in FIG. 2 is externally applied which has a sufficient duration for DC regeneration of any one of the bubble output signal waveforms S.sub.1, S.sub.2 and S.sub.3. In synchronism with the control signal W.sub.ST, the waveforms N, S.sub.1, S.sub.2 and S.sub.3 are DC regenerated to produce corresponding waveforms N', S.sub.1 ', S.sub.2 ' and S.sub.3 ' as well known in the art. At the same time, a threshold level Vth is set which is about half the level difference between a maximum and a minimum (a positive peak of the waveform S.sub.3 ' and a negative peak of the waveform S.sub.2 ' in this example). In the discriminator 6, within the duration of a strobe signal N.sub.ST, portions of the DC regenerated waveforms S.sub.1 ', S.sub.2 ' and S.sub.3 ' falling below the threshold level Vth are converted into logical signals of high level S.sub.10, S.sub.20 and S.sub.30 as shown in FIG. 2. Obviously, the margin of the logical conversion is small for the DC regenerated waveform S.sub.3 ', and the logical high level signal S.sub.30 has the smallest duration of the three signals and is critical because if the bubble output signal S.sub.3 shifts slightly to the right in the drawing, the high level signal S.sub.30 will disappear, resulting in failure to detect one bubble output signal represented by the waveform S.sub.3. Of course, prolongation of the duration of the control signal W.sub.ST is not a practical solution to this problem in view of the resolution of the bubble output signal detection.